The extent to which peak bone mass, structure, and strength are improved by adolescent activity will be determined, using gymnastics participation as a model of mechanical loading. A 6-10 year longitudinal study of approximately 60 late-adolescent female gymnasts and non-gymnasts will be extended and supplemented by the addition of 80 early-adolescent girls (gymnasts and non-gymnasts). Annual dual energy X-ray absorptiometry scans of the forearm, proximal femur, lumbar spine, and total body, with concurrent peripheral quantitative computed tomography scans of the forearm, will evaluate skeletal geometry, density and strength. Diet, gymnastics and other physical activity, anthropometry and Tanner self-stage will be assessed semi-annually;muscle strength will be measured annually to coincide with skeletal scans. Longitudinal and mixed longitudinal designs will use Multilevel Modeling to examine the relative contributions of geometric and densitometric skeletal adaptations and determine the persistence of those benefits to skeletal maturity. This work will provide a unique longitudinal perspective of bone growth and skeletal maintenance in the context of maturation and gymnastic activity. If this extreme model of mechanical loading does not yield persistent skeletal improvements, more widely applicable modes are unlikely to confer lasting benefits. Enhancement of peak bone mass and structure is a crucial strategy for the prevention of osteoporosis. Mechanical loading appears to increase bone acquisition during growth, yet the extent to which these benefits are maintained is unclear. The proposed study will detail skeletal responses to adolescent gymnastic activity and assess maintenance of benefits to adulthood, providing a foundation for the development of an adolescent exercise prescription to improve ultimate bone health.